PAU LOCKENDEN
This month, Paul dives into software-defined radio, before attempting to get his head round some scary signal processing.
There are many different ways to watch live TV on your computer. The simplest is probably just to stream it over the internet, either from the broadcaster’s own website (such as BBC iPlayer), via your service provider (Sky Go, for example) or even through a third-party website such as tvplayer.com. Plus there are any number of dodgy streams for “premium” events such as top-end football matches and pay-per-view boxing, although these are usually hosted on websites containing all the latest zero-day browser exploits and trojans. Unless you fancy spending the next couple of days disinfecting your PC and reinstalling everything, avoid such pirate streaming sites – in many cases, it works out cheaper just paying for the content!
If streaming isn’t an option (perhaps you have ropey broadband at home, or maybe you are camped out in a field somewhere using a datacapped mobile connection), the next best option is to plug a TV receiver into your computer. But what type? Although you can buy plug-in satellite receivers, that also involves setting up a dish – probably not something you’d want to do if you’re backpacking with a tent. The alternative is a terrestrial TV receiver, and because the analogue signal was switched off some time ago,
this needs to be DVB (Digital Video Broadcasting), although most people just call it Freeview, which is the UK’s DVB platform.
There are loads of different DVB receivers available for PCs. A few of them are standalone boxes that you connect via a lead to a USB port on your computer, but the vast majority are dongles – they look like fat memory sticks. Some have a flip-up antenna, but you’re better off finding a model that uses an external aerial: you’ll get a stronger and less noisy signal if the antenna is further away from the (electrically) noisy PC.
In the old days, any radio or TV receiver was built from a load of standard circuit building blocks such as tuners, mixers, filters, amplifiers, detectors, modulators and demodulators. RF design was a real art. It still can be in certain high-end kit, but many consumer devices now use software-defined radio (SDR) techniques instead. As the name suggests, this replaces many of these old hardware-based analogue radio building blocks with software. And guess what? Most DVB dongles are based around SDR designs.
In fact, most of them use the same hardware: a Realtek RTL2832U analogue to digital converter, and either an Elonics E4000 or Rafael Micro R820T tuner. All the actual decoding is then done in software from the I/Q data provided by the RTL2832U chip.
Testing your I/Q
So what is this I/Q data? It’s nothing to do with passing a Mensa entrance test, although as we’ll see it can be a bit of a headscratcher. The term I/Q in this context is an abbreviation for “inphase” and “quadrature”, but don’t worry about that for the moment.